1. Field of the Invention
The rapidly rising demand for energy in many countries requires an increasing level of importation of liquefied natural gas (LNG). This invention relates generally to LNG import terminals that are located offshore in water depths suitable for ship navigation. More specifically the invention concerns an LNG import terminal of open frame construction that can weathervane about a rotatable mooring structure at one end and can be rotated away from or toward a path of a docking carrier vessel to the terminal in response to operation of thrusters located at the opposite end of the terminal. Still more specifically, the invention concerns an offshore docking facility that is used advantageously in conjunction with the underground storage of hydrocarbon gas either in salt dome caverns or in depleted sulfur domes.
2. Description of the Prior Art
A common example of prior docking arrangements for two vessels at sea is the side-by-side mooring of two conventional hull vessels, i.e., mooring the carrier vessel to a converted oil tanker hull. Such an arrangement is disclosed in U.S. Pat. No. 6,546,739 of Frimm, et al issued Apr. 5, 2003. The converted oil carrier has an LNG regasification plant mounted thereon and is moored to an external single point mooring buoy. Such a converted hull vessel is commonly used offshore, but is limited to relatively benign sea-states because of excessive relative motion between the terminal vessel and a carrier vessel secured to its side. Larger sea-states cause large forces to occur between the vessels and pose a significant safety risk to the operation. Not only do both vessels react individually to the environmental loads, there is a coupling effect between the two vessels that may amplify the motions. This coupling action makes the prediction of the vessel motions and forces difficult with existing analytical numerical methods.
Numerous US and foreign patents describe a multitude of side-by-side vessel loading methods, and several variations of floating LNG regasification units. The following patents and published applications show various side-by-side loading arrangements and methods: US 2003/0206771, of Poldervaart, on Nov. 6, 2003; WO 03/093099 A1, of Poldervaart on Nov. 13, 2003; WO 03/049994 A1, of Wille on Jun. 19, 2003; WO 03/033341 A1, of De Baan on Apr. 24, 2003; U.S. Pat. No. 6,546,739, of Frimm et al. on Apr. 15, 2003; U.S. Pat. No. 4,494,475, of Tor Eriksen on Nov. 1, 1982; U.S. Pat. No. 4,317,474, of Kentosh on Mar. 3, 1980; U.S. Pat. No. 4,098,212, of Kemper on Feb. 17, 1977; and U.S. Pat. No. 3,908,576, of Van der Gaag on Sep. 30, 1975.
3. Objects of the Invention
The primary objects of this invention are to provide:
a. An offshore floating import terminal for the purpose of offloading LNG carrier vessels and for and the purpose of pressurizing and warming the LNG to a dense phase gas state prior to transfer of the gas to a subsea gas pipeline and/or to an underground storage cavern.
b-1. An improved offshore floating import terminal as described in paragraph (a) above, except that: (1) the warmed gas is exported from the floating terminal to only a sales gas pipeline; (2) no LNG or gas storage is provided off of the floating terminal; and (3) the floating terminal does not have significant on-board storage of LNG.
b-2. An improved offshore floating import terminal as described in paragraph (b-1) above, except that the floating terminal does have significant on-board storage of LNG transferred from a carrier vessel, where LNG is applied to regasification equipment on the floating import terminal from on-board storage tanks.
c. An improved offshore floating import terminal as described in paragraph (a) above, except: (1) the warmed gas is exported from the floating terminal to only a sales gas pipeline; (2) large insulated tanks with a capacity of at least 20,000 m3 of LNG are provided on board the floating terminal; and (3) no LNG or gas storage is provided off of the floating terminal before the gas reaches the coastal shoreline.
d. An improved offshore floating terminal facility for the purpose of offloading LNG carrier vessels at LNG transfer rates of at least 1500 m3/hr and scalable for offloading rates upward of 15,000 m3/hr in a side-by-side (SBS) mooring arrangement.
e. An improved offshore floating terminal facility for the purpose of offloading LNG carrier vessels at LNG transfer rates of at least 1500 m3/hr and scalable for offloading rates upward of 15,000 m3/hr in a side-by-side (SBS) mooring arrangement, wherein conventional LNG loading arms are used for transferring LNG, and wherein utilization of the conventional loading arms do not require substantial modification of the LNG carrier's cargo side manifold piping where conventional loading arms are used such as those presently manufactured by FMC Loading Systems of Sens, France.
f. A dock structure that, because of its open frame construction, minimizes the relative motions between the floating dock and the moored LNG carrier such that relative motions are less than would occur between two conventional vessel hulls connected together in a side-by-side arrangement.
g. A floating structure that due to its inherent design has substantially less motion than an equal length conventional hull (such as a converted oil tanker hull) when subjected to environmental forces acting on the floating body.
h. A structural arrangement that minimizes the coupling effects between the dock structure and the SBS moored LNG carrier, and has substantially less relative motion than would occur between two conventional hull vessels moored side-by-side.
i. A floating terminal facility that is single point moored by an internal mooring turret, thereby allowing weathervaning with the environmental forces of wind, waves and sea current where the internal turret is located at an optimal point aft of the forward end of the dock, the distance from the forward end being in a range between about 0% to 30% of the dock overall length.
j. Powered maneuvering capability of the dock to facilitate a safer approach and side-by-side mooring of the LNG carrier to the dock where reversible marine thrusters on the aft end of the dock serve to swing the dock around the single point mooring.
k. A floating terminal facility with                (1) an internal turret mooring located near one side of the dock structure, near the side at which the LNG carrier vessel is moored, i.e., not located near the centerline of the dock and not located near the centerline of the moored LNG carrier;        (2) a ship pull-in hawser fairlead located at a point on the forward end of the dock and near the dock's side adjacent to the LNG carrier so as to improve the operational safety of mooring the ship to the dock;        (3) all of the power generation and process equipment is mounted on the floating terminal; and        (4) a diffuser for the heat exchanger cold seawater water outlet arranged to discharge in a way that provides (a) beneficial thrust force to assist the dock structure in remaining in contact with the LNG carrier, and (b) to discharge the cold sea water transverse to the platform and to the current in order to improve disposal of the cold sea water.        
1. An improved offshore floating import terminal with an open. frame construction including a column stabilized floating platform, a type construction known in the offshore industry for the construction of semi-submersible drilling platforms, but with dimensions and locations of the buoyant columns and pontoons arranged and designed specifically to provide enhanced floating stability and reduced motions of the platform as compared to those of a conventional shape.